leaving off closing delimiters in css [duplicate]

This question already has an answer here:

• Leaving out the last semicolon of a CSS block 11 answers

I have noticed that in a css rule, I can leave off the last semicolon and it still works

p {
color:black;
font-weight:bold;
font-size:17px
}

also, If I am commenting out something at the end of an inline style, I can leave off the closing */ and it still works

<p style="color:red;/*font-style:italic">Red</p>

Is this valid syntax, or just the web browser gracefully handling my messy code?